A more conservative approach

The power engineering division of the world's largest company makes no bones about it: the challenges of wind turbine engineering are tough and nowhere more so than in gearbox design. "Our respect for wind turbine technology has grown tremendously," says Erik Gebhardt, who in March transferred to GE Energy's wind turbine business to become its general manager. "The practical side is so complex and forces are so dramatic. We would never have imagined how complex turbines are."

GE uses gearboxes mainly from two suppliers, the Winergy business of German gearbox manufacturer Flender, owned by Siemens, and from Bosch Rexroth, a German supplier of drive and control equipment with more than 26,000 employees worldwide. Although GE manufactures gearboxes for other industries, it openly admits that making gearboxes for wind turbines are currently beyond its capability.

"We have to put the knowledge in the gearbox manufacturers' hands. It would be better if we designed the gearbox and they built it, but we don't have all the knowledge," says Vincent Schelling, chief engineer at GE's wind unit. The company, however, has put considerable time and effort into understanding what is required of a wind turbine gearbox and what can go wrong. "The specification documentation for wind turbine gearboxes is now much bigger than it was," says Schelling.

GE is not following the wind turbine design route chosen by some of its rivals, Vestas among them, to cut machine weight, and thus cost, by eliminating the thrust bearing from the drive train. Schelling says removal of the thrust bearing requires the gearbox bearings to withstand both thrust as well as torque loading. Not only that, while torque loads can be simulated in accelerated lifetime testing of gearboxes, it is impossible to simulate thrust loads. GE's prototype 2.5 MW turbine and a planned 3 MW unit include two main bearings in the drive train "preventing bending and thrust loading produced in the rotor from impacting the gearbox."

For how long GE will continue to buy its wind turbine gearboxes from outside the GE empire remains to be seen. With its use of Winergy gearboxes, the mighty GE is now in the position of being reliant on a sub-division of its arch rival, Siemens. "We have a long relationship with [Winergy] and with all this history it's a question of whether we should discuss future co-operation. It will continue on a short term basis and we must see if it can expand in the longer term," says Gleitz. "We're always looking at the make versus buy equation and no decisions have been made yet."